The present invention relates to a method and an apparatus for controlled bidirectional feeding of particulate material. This invention relates more specifically to a bidirectional feeder for delivering solid material or materials in desired proportions from one or multiple inlets simultaneously to at least two feeding points. Such bidirectional feeders generally consist of a housing, having one or more inlets, a chain and flight conveyor, and at least two outlets from the housing.
Modern combustion or gasification processes, e.g. in circulating fluidized bed reactors, can utilize a wide range of solid fuels. Different types of fuels can be burnt or gasified simultaneously or alternatively. Properties of the fuels (even properties of one particular fuel) may, however, vary greatly. Therefore a fuel feeding system which is capable of reliably handling varying types or amounts of fuels, has long been needed.
Large combustion or gasification chambers require at least two fuel feeding points in order to ensure stable and efficient combustion or gasification and low emissions. A fuel feeding system that is able to deliver fuel at two feeding points simultaneously is therefore desired. The fuel feeding rate to each feeding point should be easily adjustable for different kinds of fuels in order to be able to control and optimize the combustion or gasification and emission performances.
Especially large circulating fluidized bed reactors often have two gas outlets in the upper part of the reactor chamber and two parallel cyclones for separating solids from the discharged gases. The separated solids are recycled distinctly from each cyclone back to the lower part of the reactor chamber, normally through a loop seal. Fuel is usually fed into the system through feeding points in the loop seal. In many cases, for layout reasons for example, it is desirable to first bring the fuel to a location somewhere in the middle between the two loop seals and only thereafter divide the flow of fuel between the two feeding points in the loop seals.
Various systems have been used for feeding solid fuels to two feeding points at the same time. Currently bidirectional screw feeders and drag chain conveyors are used for feeding solid material from one location in two opposite directions. Existing conveyors have some disadvantages. For example, it is difficult in bidirectional screw feeders, used to feed fuel from a hopper into two different feeding points, to reliably control the separate flows of material to each outlet. One reason is that screw feeders tend to block easily. Further, support of long screws has proven to be difficult, which leads to excess wear of bearings. Simultaneous feeding of different solid materials with the screw feeder without mixing in advance is difficult as the screw feeders have only one inlet.
In general, drag chain conveyors (chain and flight conveyors) have proven to be a relatively reliable solution to the problem of conveying different kinds of fuels. Existing drag chain conveyors do not, however, meet all requirements for reliably conveying fuel to two feeding points. Most drag chain conveyors have an inlet located at the end of the conveyor, which does not allow fuel to be fed from the inlet in two opposite directions, e.g. from a location in the middle between two loop seals to both loop seals.
Solid material delivered from a hopper through a vertical downwardly directed duct on top of a drag chain conveyor is not automatically spread evenly on the conveyor. The material tends to flow unsteadily in the duct, flowing along one side or the other of the duct. Pressure differences in the system easily cause changes in the material flow. Even relatively small changes in the composition of the solid material may also change the flow properties of the solid material. It is therefore very difficult to divide the flow of solid material evenly into two or more predetermined flows immediately at the outlet of the duct, or at the inlet to the conveyor housing. The total amount of solid material, as well as, proportions of the individual, divided, solid material flows, will vary.
The present invention addresses the problem of feeding solid material in two opposite directions and in predetermined proportions or controlled amounts, e.g. into combustors or gasifiers or some other systems needing a controlled feed of solid material to at least two feeding points.
According to one aspect of the present invention a feeder for bidirectional feeding of solid material is provided. The feeder comprises: A housing having an upper portion, and at least one inlet for solid material in the upper portion. A drag chain conveyor with flights for conveying solid material longitudinally in the housing, the conveyor being primarily disposed within the housing, and having an upper run and a lower run. An intermediate plate is disposed between the upper run and the lower run and cooperates with the flights of the upper run so that the conveyor conveys solid material along the intermediate plate in a first direction. A bottom plate associated the housing cooperates with the scrapers of the lower run so that the scrapers of the lower run convey solid material along the bottom plate in a second direction opposite the first direction. At least first and second outlets for solid material from the housing are provided, the outlets spaced from each other in the first direction, and disposed on opposite sides of the at least one inlet. And, means defining at least one opening in the intermediate plate between the at least one inlet and the first outlet for allowing a portion of the solid material being conveyed in the first direction to flow through the opening to the lower run of the conveyor, is provided. The dimensions of the opening in the intermediate plate are adjustable to control the amount of solid material that falls from the upper run to the lower run.
The intermediate plate is preferably arranged under the inlet(s) so as to receive all solid material flowing into the housing through the inlets. The intermediate plate is further arranged to reach from the inlet(s) substantially to a first end of the conveyor, thus enabling transport of material from the inlet(s) to a first outlet in the conveyor. The width of the intermediate plate is approximately the width of the housing, thus preventing solid material from falling down at the sides of the upper run of the conveyor.
The opening in the intermediate plate is e.g. adjustable perpendicularly to the running direction of the conveyor, being approximately 30-70% of the total width of the intermediate plate. In most cases, when a solid material flow has to be divided into two equal portions, the width of the opening will be about 50% of the total width of the intermediate plate. The width of the opening may, however, depend on e.g. location of solid material inlet(s) and the location of the opening in the intermediate plate.
The opening may be arranged at either side of the intermediate plate, or two openings may be provided--one at each side of the intermediate plate. The opening can alternatively be arranged in the middle of the intermediate plate. The form of the opening may be rectangular or triangular or any suitable form having a size which can conveniently be adjusted.
Different arrangements for controlling the size of the opening (or rather the amount of solid material flowing through the opening down to the lower run of the conveyor) may be provided. Preferably a gate valve is arranged to shut off or widen the opening. The gate valve may, e.g., be a sliding plate arranged below the intermediate plate in connection with the opening. The position of the plate may be controlled to shut off one part of the opening or the whole opening.
The invention allows the solid material to be divided into two predetermined proportions at a distance from the inlet(s), thus giving the solid material time to form a continuous and even layer of solid material on the upper run before it is to be divided and led to the outlets.
A spreader may be arranged between the solid material inlet(s) and the adjustable opening above the upper run of the conveyor in order to help level out the solid material flowing on the intermediate plate. The spreader ensures an even flow of solid material along the intermediate plate and makes division of solid material in predetermined proportions easier. The distance needed to spread the solid material evenly on the upper run is shortened if a spreader is used. The spreader may also be used to control the total flow of solid material from the inlet(s) towards the adjustable opening as it may form a barrier allowing only a predetermined maximum flow of solid material to flow along the intermediate plate. The spreader may be adjustable to allow more or less material to flow along the upper run. Different types of spreaders are known and can be used. The spreader may be a beam transversely arranged above the conveyor, or it may have the form of a plough.
According to another aspect of the present invention, a circulating fluidized bed reactor is provided. The reactor comprises: A combustion chamber. A pair of ducts for feeding solid particulate material into the combustion chamber adjacent the bottom thereof. Means for simultaneously feeding solid material to be introduced into the combustion chamber to the ducts, the feeding means comprising: a housing having an upper portion, and at least one inlet for solid material in said upper portion; a drag chain conveyor with flights for conveying solid material longitudinally in the housing, the conveyor being primarily disposed within the housing, and having an upper run and a lower run; an intermediate plate disposed between the upper run and the lower run and cooperating with the flights of the upper run so that the conveyor conveys solid material along the intermediate plate in a first direction; a bottom plate associated with the housing cooperating with the scrapers of the lower run so that the scrapers of the lower run convey solid material along the bottom plate in a second direction opposite the first direction; at least first and second outlets for solid material from the housing, the outlets spaced from each other in the first direction, and disposed on opposite sides of the at least one inlet; and, means defining at least one opening in the intermediate plate between the at least one inlet and the first outlet for allowing a portion of the solid material being conveyed in the first direction to flow through the opening to the lower run of the conveyor. One of the outlets is connected to each of the ducts.
According to still another aspect of the present invention, a method for controlled feeding of solid material bidirectionally is provided. The method comprises the steps of: Introducing solid material into the opening of the housing to fall down onto the intermediate plate. Conveying the material with the top run on the intermediate plate in the first direction toward the first outlet. Adjusting the size of the opening in the intermediate plate to proportion the amount of material that will fall through the opening to the bottom plate. And, conveying the material on the bottom plate with a lower run of the conveyor to the second outlet. The method may also comprise the step of providing adjustable spreading action to control the solid material flow on the upper run of the conveyor.
It is the primary object of the present invention to provide an effective method and apparatus for delivering solid material to two or more different feeding points, in which the disadvantages of the prior art are minimized. This and other objects of the invention will become clear from the detailed description of the invention and the appended claims.